Ceramic tiles are commonly formed by pressing material in powder form, of between 1% and 10% moisture content, within a mould. This forming method is commonly known as dry forming. The soft material is loaded into the mould by known means. After the mould has been closed by punches operated by the pressing members, the powder undergoes initial light pressing, with consequent volume reduction, facilitating powder deaeration. The inital light pressing, also known as a first pressing, is followed by the deaeration stage, during which pressing is interrupted and the mould is sometimes reopened to allow the air to escape. The light pressing subjects the powder to a pressure which is about one tenth of the pressing pressure. This is followed by the main pressing to a pressure of about 400 kg/cm.sup.2, which ensures perfect powder compaction.
The main pressing generally takes place in several successive steps at increasing pressure up to the maximum pressure. The thrust exerted by the upper cross-member of the press is distributed over the total surface of the tiles pressed during each cycle.
It should be noted that each time reference is made to the term "pressure" in the text, this unless otherwise specified means the compacting pressure to which the powder is subjected within the forming mould. The largest currently available presses have a capacity (pressing force) of 4000 tones, and during each cycle are able to press a surface area of not exceeding 10,000 cm.sup.2 ; Thus, for example, they can operate a die having three impressions of 54 cm.times.54 cm.
Tiles of large and ever larger dimensions, having sides exceeding one meter, have not been able to be formed so far by known dry processes because the capacity (pressing force) of the press required to compact the pressure would involves a structure of such dimensions as to be difficult to construct.
Large-dimension tiles having sides of the order of one meter or more are currently manufactured either by extrusion processes or by wet forming processes within hygroscopic moulds similar to those used for sanitary appliances.
Apart from the low cost effectiveness of such processes, the subsequent high-temperature firing of the material creates important problems due to the excessive or poorly distributed moisture contained in the material.